That trailer-description might be fake, but it's entirely possible that Jonathan wont be the super-friendly all-saint he's usually depicted as. He might be more of a realist and pessimist while being overprotective as well. Kinda like in Birthright.
So him saying that line to Clark might be a pessimistic way of saying he wont be able to save everybody and that he's in over his head in this world. Though that line makes it sound like he shouldnt even try, which is stretching it.

__________________"Anyone who has never made a mistake has never tried anything new"